1. Field of the Invention
The present invention relates to a developing sleeve for a magnetic brush developing unit used in an image forming apparatus such as an electrophotographic copying machine or the like.
2. Description of the Prior Art
In an image forming apparatus such as an electrophotographic copying machine or the like, a developing unit is used to develop an electrostatic latent image formed on a photosensitive drum. As one of such developing units, a magnetic brush developing unit is known which is provided with a cylindrical developing sleeve having a plurality of magnets contained therein, the developing sleeve being rotated to form on the outer circumferential surface thereof a magnetic brush from developer consisting of toner and carrier. In this type of magnetic brush developing unit, as the developing sleeve rotates, the magnetic brush formed thereon rubs against the surface of a photosensitive drum, which is also rotating, in a developing area where the magnetic brush faces the photosensitive drum, to develop the electrostatic latent image on the photosensitive drum with the toner in the magnetic brush.
As disclosed in Japanese Laid-Open Patent Publication No. 53-3347, such a magnetic brush developing unit is provided with a plurality of axially parallel grooves of a prescribed shape arranged with appropriate spacing in a circumferential direction on the surface of the developing sleeve. With such grooves formed on the surface of the developing sleeve, carrier particles to which toner adheres can smoothly flow to the developing area facing the photosensitive drum. This allows the carrier particles having the toner adhering thereto to circulate within the developing area, thereby enabling the toner to properly develop the electrostatic latent image on the photosensitive drum.
However, when grooves having angular corners are formed on the surface of the developing sleeve, the distance between the surface of the developing sleeve and that of the photosensitive drum changes abruptly at positions where the grooves are formed, which causes the strength of the electric field applied to the developing area to change suddenly. Furthermore, the bristles of the magnetic brush erect differently in the grooves than in the other circumferential areas of the developing sleeve. Thus, when an image is to be formed which has portions the whole area of which are colored with the toner (these portions are hereinafter referred to as "solid-color areas") such as when copying a photograph, unevenness in toner density may be caused in the solid-color areas. In particular, as the ratio of the surface speed of the developing sleeve to that of the photosensitive drum is reduced, the magnetic brush formed from the developer carried on the developing sleeve is caused to contact the electrostatic latent image on the photosensitive drum for a longer time in the developing area, thereby making the density unevenness in the solid-color areas further noticeable.